In Cambridge, Mass., not too far from the Charles River, which cuts near Harvard and M.I.T., David Pearson is attempting to build an un-hackable network.

Pearson is a division scientist at BBN Technologies, a private research company in Cambridge, Mass., which is most famous for building, in 1969, the first few nodes of a computer network connecting its headquarters to Harvard University and Boston University that over time would evolve into the Internet. Now the firm has built a network it says is impervious to hackers.

"If someone is eavesdropping, they introduce errors into the communications," Pearson says. "If that happens we just throw the keys away and start over."

For decades, public networks have done a fair job of protecting most sensitive communications from all but the most determined efforts to crack them. But the field is changing quickly. Looming on the horizon is the threat of quantum computing, which one day will render conventional data security algorithms obsolete.

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